userland: add assembly support

Move arm assembly cheat here, and start some work on x86 cheat as well.

userland/arch/aarch64/c/reg_var.c

@@ -0,0 +1,27 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#register-variables */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    register uint32_t x0 __asm__ ("x0");
+    register uint32_t x1 __asm__ ("x1");
+    uint32_t new_x0;
+    uint32_t new_x1;
+    {
+        x0 = 1;
+        x1 = 2;
+        __asm__ (
+            "add %[x0], x0, #1;"
+            "add %[x1], x1, #1;"
+            : [x0] "+r" (x0),
+              [x1] "+r" (x1)
+            :
+            :
+        );
+        new_x0 = x0;
+        new_x1 = x1;
+    }
+    assert(new_x0 == 2);
+    assert(new_x1 == 3);
+}